This project is concerned with analyzing the difference in the Na-K pump and leak of cells of mammalian hibernators which enable them to retain normal Na and K concentrations at low temperature. These aims are to be pursued in erythrocytes, which are well suited to examination of both passive and active mechanisms, and in any epithelial cell type (renal, hepatic or intestinal) which lends itself to similar analysis. While fluxes in epithelial cells are more difficult to measure quantitatively, they may have advantages over red cells in possessing dynamic regulation of pump activities (pump turnover, membrane replacement, etc.) and spatial separation of various leak pathways (brush border, basolateral surface). For each cell type the first step is to assess the separate contributions of still-active pump vs reduced leak at low temperature. After this, the details of each component are analyzed kinetically (e.g., partial fluxes in the case of pumps, solute interactions in the case of leaks) and by manipulation of membrane constituents and effectors. The general purpose of the project is twofold, both to understand the process of retention - or loss - of ionic regulation in the cold, which is a factor initimately involved with survival in the cold, and to use cells of hibernators to explore fundamental questions of transport.